The invention relates generally to combustion apparatus.
More particularly, the invention relates to a method and arrangement for reducing NO.sub.x emissions from combustion apparatus, especially large furnaces.
Steadily increasing environmental pollution has directed attention to the problem of minimizing nitrogen oxide or NO.sub.x emissions from large furnaces. The formation of NO.sub.x depends essentially on the combustion temperature and on the fuel which is used. At high temperatures, NO.sub.x formation is increasingly favored as the dwell time and excess of air increase.
It is known that NO.sub.x emissions can be reduced by maintaining the temperature in the combustion chamber, that is, in the immediate vicinity of the combustion taking place in the combustion chamber, below a specific value. This is based upon the known fact that NO.sub.x formation increases markedly above a predetermined, critical temperature. Accordingly, it is attempted to maintain the combustion temperature below this critical temperature. This may be achieved by recirculating the waste gases or increasing the excess of air.
Both of these procedures possess the substantial disadvantage that the quantity of gas to be conveyed and cleaned increases markedly. This makes it necessary to install a much larger electrostatic or dust filter, as well as fans of higher capacity, which leads to a corresponding increase in energy consumption. Recirculation of waste gases also results in a further disadvantage. Thus, special pipes are required for recirculation of the waste gases. Due to the unavoidable temperature variations which occur, these pipes frequently tend to develop leaks. Since an overpressure is required to blow the waste gases into the combustion chamber, such leaks will permit waste gases to escape into the boiler house.